How to Make It
In a large roasting pan, mix 1/2 cup of the lemon juice with 2 teaspoons of salt. Place the lamb in the pan and coat well with the lemon juice. Let the lamb stand in the juice for 30 minutes, then drain.
In a large bowl, mix the yogurt with the oil, garlic, ginger, dry mustard, cumin, nutmeg, cardamom, black pepper, cinnamon, turmeric and cayenne. Stir in 2 teaspoons of salt and the remaining 2 tablespoons of lemon juice. With a small knife, make 1-inch-deep slits all over the lamb. Coat the lamb thoroughly with the marinade, pushing it into the slits. Cover and refrigerate for at least 6 hours or overnight.
Light a charcoal grill. Remove the leg of lamb from the marinade, leaving on a thick layer. Move the hot coals to one side of the grill and place a drip pan opposite the coals. Set the lamb over the drip pan, cover the grill and cook until glazed and an instant-read thermometer registers 140° for medium, about 2 hours.
Line a platter with the onions and sprinkle with the cilantro. Place the leg of lamb on top, brush it with the melted butter and let rest for 15 minutes. Carve the lamb into slices and serve with the lemon wedges.